This invention is concerned with improving space power systems which utilize solar cells. The invention is particularly directed to an improved solar cell having a non-oxide anti-reflective coating thereon and a transparent plastic cover.
Silicon solar cells used in space power systems are coated with an anti-reflective coating and the covered with a protective glass cover. As presently constructed, the glasses are mounted using an ultra-violet sensitive adhesive which, in turn, requires the use of an ultra-violet filter on the underside of the glass. This structure results in a decrease in the current output of the solar cells.
It has been proposed that a transparent plastic material, such as a fluorinated ethylene propylene copolymer, be utilized instead of the glass cover. This plastic material is defined in U.S. Pat. No. 2,946,763. It is relatively insensitive to ultra-violet damage and can be heat bonded directly to the cell.
While the resulting structure represents an improvement over a conventionally covered cell, a fluorinated ethylene propylene copolymer covered solar cell which has been coated with silicon monoxide has shown a degree of damage when exposed to electron bombardment. The damage results initially in the lifting of the fluorinated ethylene propylene copolymer from the cell and subsequent embrittlement of the plastic material. This damage can be attributed to two mechanisms: (1) The bombardment of the FEP plastic material results in the release of some active form of fluorine which can attack the SiO coating. This causes the removal of the anti-reflection coating and subsequent separation of the FEP plastic material from the cell; (2) Oxygen released in this reaction then reacts with the free radicals of the fluorinated ethylene propylene copolymer. This causes degradation and embrittlement of the FEP plastic material.